Shellacs not for nails in bad condition so don't be too hard on yourself.
CND do shellac videos on YouTube & a written step by step guide on their website, Liverpool lashes does a good YouTube video too.
You can buy it with a manicure qualification, you'll need the matching lamp. S2 do a starter kit which works out really good value.
There's YouTube videos from CND showing you the new 5 point application method.